Generally, as a coating device for painting work objects like vehicle bodies, rotary atomizing head type coating devices have been well known in the art (e.g., see Patent Literature 1: Japanese Patent Laid-Open No. 2002-248382, Patent Literature 2: Japanese Patent Laid-Open No. Hei 4-71656, Patent Literature 3: U.S. Patent Application Publication No. 2003/0075617, Patent Literature 4: Japanese Patent Application National Publication No. Hei 8-503416, Patent Literature 5: Japanese Patent Laid-Open No. Hei 11-28391). In this connection, disclosed in Patent Literatures 1 to 3 are arrangements in which a rotary atomizing head is mounted on a fore distal end of a rotational shaft which is put in rotation, for example, by an air motor. In this instance, a mounting shaft section having a male screw is formed at a fore distal end of the rotational shaft, while a mounting tube section having a female screw is formed at a rear portion side of the rotary atomizing head. The rotary atomizing head is mounted on the rotational shaft by threading the female screw of the mounting tube section onto the male screw on the mounting shaft section.
Disclosed in Patent Literature 4 is a mechanism for mounting a rotary atomizing head on a rotational shaft, employing an annular resilient coupling member on the back side of a rotary atomizing head, the resilient coupling member having a plural number of fitting rings along its circumference, each fitting ring being formed with a ball-shaped projecting portion at a fore distal end thereof for engagement with a groove on the part of the rotational shaft. In this case, when the rotational shaft is put in rotation, a fore distal end of the resilient coupling member is spread out in a radial direction under centrifugal force to hold the rotary atomizing head on the rotational shaft with an increased clamping force, while automatically bringing the rotary atomizing head and the rotational shaft into alignment with each other.
Further, disclosed in Patent Literature 5 is a mechanism for mounting a rotary atomizing head on a rotational shaft, by way of an annular groove which is provided along the inner peripheral surface of a mounting tube section of the rotary atomizing head, for engagement with an O-ring which is provided on the outer peripheral surface of a mounting shaft section of the rotational shaft. Furthermore, FIG. 8, and the like of Patent Literature 5 show arrangements in which a mounting shaft section of the rotational shaft is provided with a tapered portion gradually tapered off toward the fore distal end, and the mounting tube section of the rotary atomizing head is provided with an inclined inner peripheral surface which is gradually spread toward its opening side in correspondence with outer peripheral surface of the tapered portion. In this instance, the rotational center of the rotary atomizing head is automatically aligned with that of the rotational shaft by pushing the inclined inner peripheral surface of the rotary atomizing head into the inclined inner peripheral surface of the rotational shaft.
By the way, in the case of a rotary atomizing head type coating device which is adapted to mount a rotary atomizing head on a rotational shaft by means of screw threads, arrangements are made such that the screw threads are tightened by rotating a rotary atomizing head in a direction inverse to the direction of rotation of a motor, for the purpose of preventing dislodgement of the rotary atomizing head off the rotational shaft under the influence of rotation of the motor. Accordingly, as the rotational speed of a motor is increased, a rotational torque is applied in such a direction as to hold the rotary atomizing head and rotational shaft to each other with a greater clamping force under the influence of inertial force. However, when the rotational speed of the motor is on decrease, a rotational torque is applied in such a direction as to decrease the force of clamping the rotary atomizing head to the rotational shaft. Therefore, when the rotational speed of the motor is dropped abruptly in the course of a paint coating operation or at the time of an emergency stop of the motor due to a machine trouble or for other reasons, the screw threads can be loosened to such a degree as to cause dislodgement of the rotary atomizing head off the rotational shaft.
Therefore, in Patent Literature 1 mentioned above, for the purpose of preventing dislodgement of the rotary atomizing head, a projection is provided on the outer peripheral side of the mounting tube section of the rotary atomizing head, and a groove through which the projection of the mounting tube section can be inserted is provided on a shaping air ring which is mounted in such a way as to circumvent the rotary atomizing head, on the front side of the mounting tube section. However, the provision of such a dislodgement preventing mechanism comprised of projections and grooves results in complication in construction, which is reflected by large restrictions in design and a higher production cost.
Further, in a case where a rotary atomizing head is mounted on a rotational shaft through screw threads, it becomes necessary to put the rotational shaft in a rotationally locked state at the time of mounting or dismantling the rotary atomizing head. In this regard, Patent Literatures 2 and 3 disclose a rotation preventing mechanism employing a rod-like lock member which is adapted to be inserted from outside into a locking hole which is on a rotational shaft. However, the provision of a rotation preventing mechanism of this sort also invites complication in construction to hinder efforts for reductions in size and manufacturing cost of the coating device.
On the other hand, disclosed in Patent Literature 4 is a coupling mechanism employing a resilient coupling member provided on a rotary atomizing head, the resilient coupling member being provided with a projecting portion to be fitted in a groove of a rotational shaft at the time of mounting the rotary atomizing head on the latter. However, in the case of Patent Literature 4, no consideration is given to rotational displacements of the rotary atomizing head relative to the rotational shaft. In addition, the rotary atomizing head is clamped to the rotational shaft only at a point between the projecting portion of the resilient coupling member and the groove of the rotational shaft.
Therefore, the coating device of Patent Literature 4 is unable to stably fix the rotary atomizing head relative to the rotational shaft and as a result may suffer from a problem of misalignment (deviation) of a center axis of the rotary atomizing head relative to that of the rotational shaft during rotation. In such a case, it is likely for the rotational shaft, which is rotatably supported by an air bearing of an air motor, to be damaged to a serious degree by contact with the air bearing.
Furthermore, disclosed in Patent Literature 5 are arrangements in which an O-ring provided on the outer peripheral side of a rotational shaft is brought into engagement with an annular groove of a rotary atomizing head, thereby pushing an inclined inner peripheral surface of the rotary atomizing head against an inclined inner peripheral surface of the rotational shaft. However, Patent Literature 5 also fails to give consideration to rotational displacements of the rotary atomizing head relative to the rotational shaft.
Therefore, in Patent Literature 5, when the rotational speed of an air motor is increased or decreased, a follow-up delay may occur to the rotary atomizing head, as a result of a failure in following up the speed of the rotational shaft under the influence of inertial force acting on the rotary atomizing head. In case a follow-up delay of this sort takes place at the time of increasing the rotational speed of an air motor to a specified speed, the rotational speed of the rotary atomizing head reaches the specified speed with a delay in time from the rotational speed of the air motor. On the other hand, at the time of decreasing the rotational speed of the air motor to a specified speed, the rotary atomizing head causes a delay to the air motor in reaching the specified speed. As a consequence, in a paint coating operation which requires varying the rotational speed of the air motor, it becomes difficult to atomize paint particles quickly in a desired size, failing to prevent degradations in quality of coatings.